\begin{pspicture}(0,-0.5)(6,6)
  \input{fig/1d-helpers.tex}
	% Box around a single Yee cell.
	\psframe*[linecolor=lightgray,fillcolor=lightgray](1.6,2.6)(3.4,4.4)
	% Dots and arrows.
	\Hright12\Eright23
	\Eup21\Hup32
	\Hleft32\Eleft43
	% Axes.
	\Taxis014 \Xaxis140  
\end{pspicture}
